[Ir4(CO)12] was used as a precursor for the prepn. of a MgO-supported catalyst with a uniquely simple metal structure, Ir4. The precursor was adsorbed on MgO from hexanes with the metal framework remaining intact, as detd. by EXAFS spectra. The surface-bound species was predominantly [HIr4(CO)11]-, which could be extd. from the surface by cation metathesis and identified in soln. by IR spectroscopy. After treatment of the MgO-supported Ir carbonyl in He followed by H2 at 300°, the sample was characterized again by EXAFS spectroscopy. The predominant surface species are clusters of 4 Ir atoms with an av. Ir-Ir distance of 2.69 .ANG..